1. Field of the Invention
This invention relates to a device for reciprocally driving a movable member. The present invention is applicable to a device for reciprocally driving original scanning means in a copying machine.
2. Description of the Prior Art
For example, in a copying machine, a movable optical system is reciprocally moved relative to a fixed original carriage to thereby optically scan an original placed on the original carriage and project the image of the original onto a photosensitive medium. Or alternatively, an original is placed on a movable carriage and the movable carriage is reciprocally moved relative to a fixed optical system to thereby optically scan the original. With any of these types of copying machine, it is usual that the original is scanned during the forward movement of original scanning means such as the movable optical system or the movable carriage. In any case, during the scanning of the original, it is desired that the scanned means be moved at a constant speed. After the scanning of the original has been completed, the original scanning means must be moved backward from its forward movement terminal point to its forward movement starting point and in that case, in order to improve the copying efficiency, it is necessary that the time required for the backward movement which is not used for the copying operation be as short as possible, that is, the speed of the backward movement be higher as much as possible than the speed of the forward movement.
The driving devices for the original scanning means as described above include the following types of device;
(a) The type in which a mechanism rotatively movable in both normal and reverse directions and using gears or the like is connected to a drive source rotatively movable in a predetermined direction and the movement thereof is selected by a clutch for normal rotation or a clutch for reverse rotation to drive the original scanning means;
(b) The type in which during the forward movement, the original scanning means is controlled by a clutch and during the backward movement, the original scanning means is driven by either the resilient force of a spring charged during the forward movement or an exclusive motor; and
(c) The type in which use is made of a reversible motor exclusively for use for the original scanning and a driven member is driven by the change-over of a switch.
Among these conventional devices, in the one which uses a clutch, the driven member can be driven at a predetermined constant speed substantially simultaneously with the connection of the clutch, but the device of this type has a disadvantage that a great shock occurs to the scanning means when the clutch is connected. Particularly in the case of continuous copying, the speed difference during the reversal of the scanning means from its backward movement to its forward movement is so great that a great deal of shock occurs. Also, the higher the copying speed, the greater the above-described shock. Such shock adversely affects the image formation. In the system of type (a) above, a strong shock also occurs during the reversal of the scanning means from its forward movement to its backward movement.
In the type which uses an exclusive motor, the rising of the motor is slow and therefore, the shock as experienced in the type using a clutch does not occur, but this type of device has disadvantages that the rising of the motor during the forward movement is slow and that the loss of time during the reversal is great and the copying efficiency is low. Also, the system of type (b) above usually uses an AC reversible motor whose number of revolutions is determined by the number of poles and therefore, the obtainable speed of the backward movement is only about twice the speed of the forward movement and moreover, this type of device is costly.
U.S. Pat. No. 3,612,679 discloses a device which uses a cam rotatable in one direction to reciprocally drive scanning means. In this device, if the configuration of the cam is suitably designed, the starting of the forward movement of the scanning means and the reversal of the scanning means from its backward movement to its forward movement can be accomplished without causing the shock of the scanning means. Also, the speed of the backward movement of the scanning means can be made high.
In this system, however, the speed of the scanning means is determined by the configuration of the cam and therefore, to enable the aforementioned optical scanning at a constant speed to be effected, the configuration of the cam must be designed precisely and machined with ultrahigh precision, and this leads to a very high cost of the device.
The above-described situation holds true not only of the device for driving the original scanning means of the copying machine, but also of the usual driving device for reciprocally driving a movable member and moreover, driving such member so that the time required for the backward movement thereof is shorter than the time required for the forward movement thereof.